1. Field of the Invention
The present invention relates generally to channel estimation in a broadband wireless access system, and in particular, to a channel estimation apparatus and method for demodulating frame control headers (FCH) and downlink-MAP (DL-MAP) in a user terminal in a broadband wireless access system.
2. Description of the Related Art
Researchers are studying the fourth-generation (4G) communication systems, which are the next-generation communication systems, to provide users with services having diverse qualities of service (QoS) at a transmission rate of about 100 Mbps.
Particularly, current 4G communication systems include a broadband wireless access communication system, such as a Local Area Network (LAN) system and a Metropolitan Area Network (MAN), secured with mobility and QoS, and which aim to provide services at a high data transmission rate. One of the representative communication systems is an Institute of Electrical and Electronics Engineers (IEEE) 802.16 communication system. The IEEE 802.16 communication system adopts an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme to support a broadband service network in a physical channel in the wireless MAN system.
The OFDM technology is a wireless communication method using multiple carriers, and it is highly efficient with respect to frequency and has a high data transmission rate, compared to a conventional communication system using a single carrier. Since data symbols are transmitted in a frequency domain according to each subcarrier in the OFDM system, there is an advantage in that compensation in a wireless channel environment is simplified into 1-tap equalization.
The wireless channel environment varies according to time in the OFDM system secured with mobility, such as the IEEE 802.16 system, and the channel estimation method should be able to track the time-varying channel ceaselessly. The time-varying channel is estimated mainly by transmitting signals of which a receiving part is already aware through subcarriers for some pilots of OFDM symbols. In this case, a channel for subcarriers that carry actual data is estimated by performing interpolation using the pilots.
In the IEEE 802.16 system, information on one downlink frame is stored in a frame control header (FCH) and downlink-MAP (DL-MAP) disposed in the foremost part of the downlink frame and then transmitted.
FIG. 1 illustrates a conventional downlink frame demodulation in a broadband wireless access system.
Referring to FIG. 1, an FCH is disposed in the foremost part of a downlink frame. In other words, a FCH is disposed at a fixed position with the same length and modulation scheme (such as Quadrature Phase Shift Keying (QPSK) 1/12) of the first Partial Use of Subchannel (PUSC) symbol behind a preamble so that it can be demodulated even through a user terminal does not have information on the received frame, such as the structure, data burst location, and length of the frame.
The DL-MAP is disposed right behind the FCH. The length of the DL-MAP and a modulation rate are variable, and information on them is included in the FCH. The user terminal determines the demodulation parameters for DL-MAP by demodulating the FCH that is allocated to a predetermined position in the fore part of a frame at a predetermined Modulation and Coding Scheme (MCS) level, and actually figures out the zone information of the entire frame, length and location of the data burst, and demodulation information by demodulating the DL-MAP based on the demodulation parameters. Therefore, failure in demodulation of FCH or DL-MAP leads to failure of the entire frame demodulation.
The demodulation of FCH and DL-MAP is affected by synchronization and channel estimation. The channel estimation is carried out using a preamble or pilot subcarriers for a PUSC symbol. The channel estimation based on the pilot subcarriers includes a Least-Square (LS) method or a Linear Minimum Mean Square Estimation (LMMSE) method. The LS method is generally used due to its simplicity although it has inferior performance to the LMMSE method.
Excellent channel estimation performance requires synchronization in time and frequency. When time is not synchronized and the starting point of a symbol is determined posterior to a guard interval, Inter-Symbol Interference (ISI) occurs and it deteriorates the channel estimation performance. In the case of the frequency synchronization, when a frictional frequency offset which is decimal-time as long as a subcarrier interval occurs, the inter-symbol interference also occurs based on the frequency offset, which leads to deteriorated performance of channel estimation.
In general environments, it is impossible to accurately synchronize time and frequency due to a time-varying channel and Additive White Gaussian Noise (AWGN), and offset occurs more or less always. However, the channel estimation performance may not be affected by determining the starting point of a symbol in a guard interval in the case of time synchronization, and a residue frequency offset which is less than 1% of a subcarrier interval in the case of frequency synchronization.
FIG. 2 illustrates conventional structures of a preamble and PUSC symbols in a broadband wireless access system
FIG. 2 shows 1024-tab Fast Fourier Transform (FFT) used in a PUSC symbol including the FCH and the DL-MAP.
In the case of the preamble, one pilot subcarrier is transmitted for every third subcarrier. In the case of the PUSC symbol, two pilot subcarriers are transmitted for every 14 consecutive subcarriers, and the location of a pilot subcarrier is changed on a two-symbol basis.
The 14 consecutive subcarriers and the two symbols are grouped and referred to as a cluster. Also, the pilot subcarriers included in the preamble are boosted with power of 9 dB, whereas the pilot subcarriers disposed in a PUSC symbol using a frequency reuse factor 1 are boosted with power of 2.5 dB.
Since the preamble has higher pilot density and signal-to-noise ratio (SNR) than the PUSC symbol, the channel is estimated better in the preamble than in the data symbol. Also, a noise reduction filter for improving channel estimation performance cannot be used for the first two PUSC symbols that include the FCH and the DL-MAP. This is because the frequency reuse factor of the two PUSC symbols is not known until the FCH is demodulated.
The frequency reuse factor is determined based on whether the subcarriers of the PUSC symbol are used in the current frame (which is a case of a frequency reuse factor 1) or whether only part of the subcarriers are used among the subcarriers (which is a case of a frequency reuse factor 3). In the latter case where only part of the subcarriers are used (a frequency reuse factor 3), noise reduction filtering cannot be performed because a cluster in the middle part is transmitted without pilots and data.
Zone boosting deteriorates channel estimation performance for the PUSC symbol disposed in the fore part of a frame. With zone boosting, the voltage of the pilots differs according to the frequency reuse factor of the PUSC symbol. As described above, the frequency reuse factor of the first two PUSC symbols can be known only when the FCH is normally demodulated. Thus, time interpolation used to improve the channel estimation performance cannot be used for the preamble and the subsequent two symbols which are transmitted in a fixed power level. Therefore, the demodulation performance is better when a channel estimation value obtained using a preamble is used for the demodulation of the FCH and the DL-MAP than when a channel estimation value obtained by using pilot subcarriers of a PUSC symbol including the FCH and the DL-MAP.
However, the influence of a residue frequency offset appears in the form of inter-symbol interference and phase rotation in the frequency domain, which is accumulated as time passes. Since the influence of the inter-symbol interference is relatively small, only the deterioration in performance caused by the phase rotation will be considered herein. A value of phase rotation becomes larger and it is accumulated on a symbol basis. A symbol has the same rotation value for all subcarriers.
In other words, when there is a residue frequency offset, a channel estimation value obtained using a preamble has a different phase from a channel estimation value obtained using pilots of PUSC symbols in the fore part of a frame that are transmitted subsequently. Thus, when the channel estimation value obtained using a preamble is used for the demodulation of the FCH and the DL-MAP, channel estimation performance deteriorates due to the phase rotation. To sum up, when the channel estimation value obtained using the preamble is applied to the demodulation of the FCH and the DL-MAP, there is an advantage with respect to the SNR, compared to using the channel estimation value obtained using the pilots of the PUSC symbols in the fore part of the frame. However, when the residue frequency offset is considered, there is little advantage due to the phase rotation.
Also, in order to apply the channel estimation value obtained using the preamble to the PUSC symbols in the fore part of the frame, not only the phase rotation but also the effects on the time-varying characteristic of a wireless channel environment should be taken into consideration. In short, up to what symbol apart from the preamble in the sequence of symbols the channel estimation value obtained using the preamble should be applied to is determined based on the coherence time of a channel. When the channel estimation value obtained using the preamble is applied to symbols corresponding to time longer than the coherence time of a channel, the change of the channel caused by time affects more than the phase rotation and deteriorates the performance.
When DL-MAP and data burst have the same length and are demodulated in the same method, i.e., Quadrature Phase Shift Keying (QPSK) 1/12 and their demodulation performance is compared, the demodulation performance of the DL-MAP is inferior. Therefore, it is required to develop an efficient channel estimation method that has the advantage of the channel estimation method demodulating FCH and DL-MAP using a preamble and the advantage of the channel estimation method demodulating the FCH and DL-MAP using pilot subcarriers disposed in the PUSC symbols in the fore part of a frame.